1. Field of the Invention
The technical field of the invention relates to UV inkjet printers, such as wide-format UV inkjet printers, to overlay a jetted color or gray image with an ultraviolet curable varnish to achieve a glossy finish.
2. Description of the Related Art
In the related art, as a printing method for forming an image based on an image data signal on a receiver such as paper, various methods have been used. Among them, an inkjet printing method is used with an apparatus with a low cost which discharges ink only to a necessary image unit, such as an inkjet print head, and performs direct image formation on a receiver.
An inkjet printer stores the printing data electronically and controls a mechanism for ejecting the drops image-wise.
In recent years, in order to form an image having excellent water resistance, solvent resistance, rub fastness and the like on a surface of a receiver, an ink jet printing method using ultraviolet curable ink, also called UV inkjet ink, has been used wherein the UV inkjet ink is cured when being irradiated with ultraviolet light by an UV radiation device (100) has been used. The inkjet printing method using ultraviolet curable ink is also called UV inkjet printing method. The advantage of UV inkjet inks in an inkjet printing method is that they are immobilized on the receiver as soon as they are cured, they can be applied to a wide range of uncoated receivers, and they produce a very robust image.
More information about inkjet printing methods is disclosed in STEPHEN F. POND. Inkjet Technology and Product Development Strategies. USA: Torrey Pines Research, 2000.
Problems with gloss homogeneity in UV inkjet printers, especially wide-format UV inkjet printers, are observed. To solve these problems a varnish is applied on the jetted image. A varnish is a transparent liquid applied to a surface for producing a glossy appearance. A varnish may also be designed to produce satin or semi-gloss sheens by the addition of “flatting” agents. These flatting agents, also often called matting agents, are particulate substances for scattering incident light rays on the varnished surface. The varnish may be applied by an UV inkjet printing method using an ultraviolet curable varnish, also called an UV inkjet varnish. Another major advantage of using an UV inkjet printing method to apply a UV inkjet varnish is that it allows variable data printing or region-of-interest (ROI) varnishing on a receiver.
EP2221183 (MGI France) discloses a varnish printer with inkjet print heads.
US2007070162 (YOKOYAMA TAKESHI) discloses an shutter system for an inkjet printer wherein the controlling is adapted to control the dimension of the irradiation zone in the direction of scanning of the inkjet print head module (300), namely the fast-scan direction.
US2013293609 (OHKAWA MASAKATSU) discloses an UV radiation device which comprises UV LED's wherein the UV LED's are controlled to adapt the amount of irradiations.
US2008012919 (SUGUHARA HIROTO) discloses a method wherein two irradiation zones are created: one in the image formation area and one in the adhesion area.
In recent years, UV inkjet printers, such as wide-format printers, have the ability to jet colored UV inkjet inks, such as cyan, magenta, yellow, black and/or white on a receiver together with an UV inkjet varnish.
Typically applying a varnish requires for the UV inkjet varnish to be given enough time to flow (spread) across the receiver to create an even glossy surface finish before it is cured. Therefore in a typical UV inkjet method by applying an UV inkjet varnish on a color or gray image jetted on a receiver, the UV radiation device has to be offset in the slow scan direction in order to give it the extended time for the varnish to properly spread. Or the curing of the jetted UV inkjet varnish, which is not cured in the first passing of the receiver under the inkjet print head module, has to be applied in a second passing of the receiver under the inkjet print head module which is less economical due to a lower production time.
The offsetting and repositioning of the UV radiation device may cause problems such as curing the ink on the nozzle-plate of the inkjet print heads in the inkjet print head module while offsetting and failures in correct (re)positioning of the UV radiation device. The production and alignment of the means for offsetting the UV radiation device are also less economical due to the manufacturing time and manufacturing cost.
Therefore a solution is needed to lower the production time, manufacturing time and manufacturing cost and preventing occasional curing of ink on the outer surface and the inner surface of the nozzles of the inkjet print heads in the inkjet print head module.
To achieve a better print quality, time-to-cure of an UV inkjet ink may be controlled, which may be controlled by UV LEDs but the manufacturing costs and the cost of UV LEDs is economical a disadvantage. Therefore a solution is needed to control the time-to-cure with an economical advantage.